The development of high-speed, high-efficiency light sources is important in many applications. Optical data transmission, for example, can benefit from light emitting devices with increased speed and high quantum efficiency.
Quantum efficiency is defined as the ratio of the number of photons emitted in a desired direction to the number of charge carriers injected.
The performance of any light emitting device is judged by the speed and efficiency of the device. While the efficiency can be improved by better device design and/or better material growth processes, the speed is often limited by fundamental material properties. The speed of a device made of a given material is limited by the radiative lifetime of the charge carriers (electrons and holes), i.e., how fast the electrons and holes can recombine (across the band gap) to form a photon. The GaAs-AlGaAs system, for example, has a radiative lifetime of approximately 1 nanosecond.
In most light emitting devices, the electrons and holes must `thermalize` before they can radiatively recombine, i.e., emit light. Thermalization is the process of charge carriers losing excess momentum and energy in the form of heat (phonons) to the surrounding crystal lattice. Excess energy or momentum in the charge carriers can prevent radiative recombination. Conventional thermalization allows charge carriers to lose an arbitrary amount of energy through the scattering of multiple acoustic phonons of arbitrary energy. This process is generally slow and is often a greater speed limitation than the radiative lifetime.
Solid state lasers have been used extensively as light sources, particularly where high speed is required. Such lasers have the disadvantage of having an emission threshold. The emission threshold is the result of the necessity of having a population inversion in the lasing medium. An emission threshold results in energy loss and nonlinear behavior which can complicate device operation. Therefore, it would be advantageous to have a light source with the speed of a solid state laser without the emission threshold.
In recent years, researchers have been investigating the use of quantum well (QW) semiconductor structures in optoelectronic devices such as light emitting devices.
QW structures are small regions in a semiconductor crystal with a different composition than the surrounding crystal. Quantum wells are planar. The QW has a different band-gap structure than the surrounding material and so can confine electrons or other particles (e.g., holes) within it. When confined to such a small region of space, particles can exhibit strongly wavelike properties. Most significantly, a confined electron can only have certain, discrete energies. These energies are determined by the physical dimensions of the QW and the intrinsic properties of the QW material and the surrounding material. Other particles or combinations of particles such as holes or excitons (bound electron-hole pairs) can be similarly affected by a QW. Proper selection of the QW geometry and material composition (of both QW and surrounding material) allows one to build QW structures with predetermined energy states for electrons, excitons, holes, and other particles.
Electrons and holes can be manipulated to travel between these energy states and therefore emit and detect photons of certain, predetermined energies.
U.S. Pat. No. 5,588,015 to Yang discloses light emitting devices based on electronic transitions between electron energy states in two closely spaced quantum wells (QWs). Electrons traveling from a first QW emit light as they step down to the lower energy state of the second QW. This invention requires two closely spaced QWs to operate. Also, the QWs must be made of dissimilar materials such that the conduction band edge of the first QW is lower in energy than the valence band edge of the second QW. Electrons are injected to the first QW using the well known technique of resonant tunneling.
U.S. Pat. No. 5,610,413 to Fan et al discloses a light emitting device exploiting electronic transitions in at least one QW and preferably multiple quantum wells separated by multiple barrier layers. The device of Fan requires that the energy of QW-bound excitons exceeds the energy of longitudinal optical phonons. This condition enables the excitons to exist at room temperature and also enables higher photon production (light output). It is notable that this invention does not tune the confined electronic energy states to match the energy of longitudinal optical phonons. Also, it is notable that this invention does not use a micro-optical cavity. The thermalization process exploited in this invention is relatively slow, and therefore slows the light emission of the device.
U.S. Pat. No. 5,023,879 to Wang et al discloses an optically pumped QW IR light source. This invention uses an asymmetric stepped QW to provide at least three electron energy states. The energy state transition energies are selected such that certain nonradiative recombinations are fast (via optical phonon scattering) and certain nonradiative recombinations are slow (via acoustic phonon scattering). Specifically, faster transition rates are facilitated by transition energies larger than the optical phonon energy of the QW material. Transitions with energies lower than the optical phonon energy have slower recombination rates due to their dependence upon acoustic phonon scattering. This difference in recombination rates facilitates the production of a population inversion in the QW, which allows for lasing. This device is necessarily optically pumped, which greatly increases the complexity and expense of an operating device.
None of these prior art devices provide a QW light emitting device with the speed and quantum efficiency that is possible with QW structures.